Thin-Entrance Window Sensors for Soft X-rays at LCLS-II

Free electron lasers, such as SLAC's LCLS-II, will provide unique scientific imaging opportunities. In order to fully utilize these facilities, we need to develop detectors with shallow entrance windows that will enable detection of soft x-rays from 250eV-1.5KeV. Achieving adequately shallow entrance windows is challenging because the high temperature anneal used to active the dopant for the diode termination also drives the dopant profile deeper, growing the region that is insensitive to soft x-rays. A new microwave annealing technology provides an efficient way to achieve shallow entrance windows in fully depleted high-resistivity silicon sensors. The new technique can activate dopants at low substrate temperature, with minimal dopant diffusion. We implanted test wafers with low energy (10KeV) Arsenic implant, followed by microwave annealing with the new technique. SRP and SIMS measurements were used to verify dopant activation with negligible dopant diffusion. We then applied the microwave anneal process to a planar sensor device wafer, using the new process to create the backside diode contact. Electrical test of the resulting sensors shows good reverse bias diode characteristics. The sensors have been bump-bonded to a read-out ASIC and used successfully to measure an Fe-55 x-ray spectrum.